A nitride semiconductor such as GaN, AlN, InN and the like and a material of a mixed crystal thereof may have a wide band gap, so as to be used as a high-power electronic device or a short-wavelength light-emitting device. Among these, research and development has been conducted on the technologies of a Field-Effect Transistor (FET) and especially a High Electron Mobility Transistor (HEMT) as a high-power device (see, for example, Japanese Laid-open Patent Publication No. 2002-359256).
The HEMT using such a nitride semiconductor is used in a high-power and highly-efficient amplifier, a high-power switching device and the like.
For example, the band gap of gallium nitride (GaN), which is one of the nitride semiconductors, is 3.4 eV, which is greater than the band gap (1.1 eV) of Si and the band gap (1.4 eV) of GaAs, so that GaN has higher breakdown field strength. As the HEMT using GaN, there is a HEMT in which the electron transit layer is formed of GaN, the electron supply layer is formed of AlGaN, and an aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure is formed.
Due to the AlGaN/GaN heterostructure, a piezoelectric polarization may be excited due to lattice distortion caused by a difference in lattice constants between AlGaN and GaN. Due to the excited piezoelectric polarization, in the GaN layer, highly-concentrated Two-Dimensional Electron Gas (2DEG) may be generated in a region near the interface (boundary surface) between the AlGaN layer and the GaN layer.
The HEMT using GaN is thought to be used especially in a high-efficiency switching device and a high withstand voltage power device to be useful in an electric vehicle or the like. In such a high withstand voltage power device, from a viewpoint of the circuit design, it is desired to support a normally-off operation.